Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/403—Cells and electrode assemblies
  • G01N27/404—Cells with anode, cathode and cell electrolyte on the same side of a permeable membrane which separates them from the sample fluid, e.g. Clark-type oxygen sensors
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
  • H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture

Definitions

• the present invention relates to sensors capable of detecting a gas in a fluid and more particularly to sensors for detecting the presence and the amount of oxygen (0 2 ).
• a number of electrode assemblies are commercially available which may be used to detect the presence of oxygen in a gaseous or liquid medium.
• U.S. Patent No. 3,449,231 (Adams et al.)
• U.S. Patent No. 3,577,332 (Porter et al . )
• U.S . Patent No. 4,078,981 (Neti et al.)
• U.S. Patent no. 4,268,370 relate to various designs and modifications of such sensors.
• these sensors comprise assemblies for the determination of the partial pressure of oxygen, or other constituents; in the dissolved or gaseous state in a fluid medium utilizing a thin membrane, such as polyethylene, which is permeable to the constituent being analyzed in the fluid medium.
• the assemblies comprise a cathode and an anode, both of which are normally formed from a noble metal such as platinum, gold, silver, rhodium or the like, connected by an electrical circuit which includes a source of power and a suitable measuring instrument, such as an ammeter.
• the electrical circuit between the cathode and the anode is completed by a suitable electrolyte which is normally disposed as a thin layer between the membrane and at least the cathode.
• the body of the sensor defines a reservoir for the electrolyte which communicates with the thin layer so as, depending on the location of the anode, to complete the circuit between the cathode and the anode.
• oxygen passing through the membrane is reduced at the cathode when a suitable polarizing voltage is imposed across the electrodes and this reaction causes a current to flow between the electrodes that is proportional to the partial pressure of oxygen in the medium being analyzed.
• Such passage of oxygen through the membrane causes a change of the steady state of oxygen reduction and oxygen generation at the electrodes, thus producing a measurable change in the current flow between the electrodes until a steady state is again established .
• the change in current flow is directly related to the concentration of oxygen in the medium being analyzed.
• Sensors of the types described above are often subjected to severe operating conditions, such as where the medium being analyzed is maintained at elevated temperature or where it is desired to subject the sensor to steam sterilization and it is highly desirable to protect the electrodes, normally by fusing the electrode in a protective glass sheath, except for a defined area which is exposed to electrolyte contact. Under such operating conditions it is essential that the sheath remain free of cracks and leaks to prevent any reactions at the electrode, except at the exposed area, which will produce excess current and result in erroneous readings.
• small diameter rhodium wire (less than .01 inch), which is resistant to CO2 encountered in certain high temperature applications, can be fused with some degree of success in lead glass in the construction of cathode assemblies for oxygen sensors which may be exposed to high temperature operations.
• Plastic sealed electrodes have also been used and have been found to be subject to many of the same problems encountered with the glass sealed electrodes due to failure of the sealing materials when exposed to caustic electrolytes at elevated temperatures.
• an improved glass sealed electrode assembly and method of manufacture in which the electrode is resistant to thermal shock and has an increased useful life even under severe operating conditions.
• an electrode assembly which comprises a thin, disc-shaped noble metal electrode onto one surface of which is spot welded a conductive wire.
• the electrode and conductive wire are fused in a tubular glass sheath and the surface of the metal electrode opposite the surface to which the conductive wire is connected is exposed for contact with the electrolyte.
• the electrode is preferably formed from a noble metal foil such as gold, platinum, rhodium and alloys thereof .
• the conductive wire need not be of the same composition as the metal electrode and need not be formed from a noble metal, although it is preferred that the conductor also comprise a noble metal that will make a good s ' eal with the glass.
• the choice of glass used to form the sheath of the electrode assembly of this invention is not critical and may comprise any of the glass compositions conventionally used for fused glass electrodes, such as lead glass, soda lime or the like .
• the electrode assembly consists of a rhodium cathode and a platinum conducting wire, both of which are disposed in a soda lime glass body.
• a second electrode consisting of an annular platinum body is fused to the glass body and is concentrically disposed with respect to and in close proximity to the rhodium electrode and serves as the anode in any oxygen sensor.
• the electrode is formed as a disc or button cut or stamped from a noble metal foil and the conducting wire is spot welded to one surface of the button.
• the free end of the wire is drawn into the bore -of a glass tube, preferably a capillary tube, with the metal button abutting the end of the glass tube which has been blown into a cone shape to keep the button centered and perpendicular to the glass tube.
• the glass tube is flame worked to permit the glass to flow around the button and the wire.
• the assembly is annealed for sufficient time to relieve stress in the glass.
• the assembly is then subjected to a grinding operation to expose a contact surface and to obtain the desired radius of curvature for the finished electrode.
• Fig. 1 is a side view, partially broken away and partially in section, of an oxygen sensor including a fused glass cathode constructed in accordance with the present invention
• Fig. 2 is an exploded side view of the components of the cathode of Fig. 1 prior to fusion;
• Fig. 3 is a side view of the cathode of Fig. 1. Description of the Preferred Embodiment
• an oxygen sensor generally designated as 10, comprising a cylindrical plastic body 12 having a cylindrical recess 14 extending substantially through the body and opening at one end thereof.
• a second opening 15 is provided in the wall of the body 12 and is closed by a plug 16 which is screw-threaded into the second opening .
• the recess 14 is closed by means of a membrane 20 which is stretched across the opening of the recess 14 and held there by a holder 22 which is clamped between the end of the body 12 and a cap 17 which is threadably engaged with the end of the body.
• the membrane is selected from a material that is permeable to the component being analyzed, in this case oxygen, and that is substantially impermeable to the electrolyte.
• suitable materials include polyethylene or Teflon.
• the cap 17 is provided with a central opening 18 for contact between the membrane 20 and a fluid being analyzed.
• a connecting terminal 24 is disposed on the body 12 and is connected in a circuit, shown generally as 26 to a source of current 27 and a current measuring instrument 28.
• the connecting terminal 24 also serves as the mounting means for a glass sheath 42 carrying a cathode 32 and an anode 34.
• the cathode 32 and the anode 34 are connected through the terminal 24 of the circuit 26 by anode conductor 36 and a cathode conducting wire 38.
• the cathode 32 is located adjacent the opening 18 of the cap 17 so that the membrane 20 is stretched over the cat_.-.-de to provide therebetween a thin electrolyte film space which is in communication with the cylindrical recess 14 of the body 12.
• the anode 34 is an annular shaped member disposed concentrically with an immediately adjacent to the cathode 32 to define therebetween an annular space 39, preferably between about .002 and .010 inches.
• a preferred electrolyte is a 2% aqueous solution of potassium hydroxide.
• the anode is platinum and the cathode is rhodium.
• the electrolyte is introduced into the cylindrical recess 14 which serves as a reservoir for the electrolyte and sufficient voltage (on the order of 0.75 volts) is imposed between the anode 34 and the cathode 32 to cause reduction of oxygen at the cathode and generation of oxygen at the anode without causing the decomposition of the electrolyte. In the absence of any substantial amount of oxygen in the electrolyte there will be little or no current flow between the anode and the cathode.
• the cathode 32 is sealed in a glass sheath in the manner taught herein to provide an improved fused glass electrode assembly that is resistant to the development of cracks in the glass sheath due to exposure to high temperature and caustic electrolyte, even though the coefficient of expansion of the cathode and the glass sheath may be dissimilar.
• a glass sheathed cathode assembly constructed in accordance with the invention consisting of the cathode 32 and the cathode conducting wire 38 which are fused in a glass sheath 42.
• the surface of the cathode 32 opposite the surface to which the conductor 38 is affixed is exposed to contact with the electrolyte and is smoothly configured to provide a uniform thin electrolyte space between it and the membrane 20.
• the cathode which may be composed of any of the noble metals that can withstand the flame working temperatures comprises a thin disc-shaped member formed .from a foil of the --desired noble metal ' .
• the foil thickness is not critical and normally is available in a range of between .01 and .025 inches in thickness.
• the cathode 32 may comprise any of the noble metals such as gold, platinum, rhodium or alloys and combinations thereof, as they are relatively chemically inert with respect to the electrolyte and most fluids and gases ton which the sensor may be exposed and they have desirable electrical characteristics.
• rhodium, or rhodium-plated material is preferred for use because of the resistance of rhodium to C0 2 interference.
• the diameter of the electrode member to be fused in a glass body must be controlled, regardless of the coefficient of expansion of the glass, in order to obtain an electrode that is reasonably resistant to stress in the glass that eventually can cause glass cracks and the resultant failure of the electrode. Accordingly, the electrode member of a fused glass electrode assembly made in accordance with the teachings of the prior art is limited in diameter to about .005 to about 0.010 inches.
• the diameter of the electrode member does not produce the stress effect on the glass as does a conventional electrode member and as a consequence the diameter of the electrode member is not so limited. Accordingly, if desired, the electrode diameter may be increased beyond the size conventionally employed in prior art devices to increase the output of the electrode.
• a close match of the coefficients of expansion of the electrode member and the glass is not critical, thus avoiding the necessity for using lead glass.
• a more stable glass for the glass sheath such as, for example, soda lime glasr to avoid the manufacturing problems encountered with lead glass.
• lead glass may be used as the composition of the glass sheath. Assembly of the glass sheathed electrode in accordance with the present invention is described herein in connection with the rhodium cathode 32 shown in Fig. 1. However, the manufacturing technique will be the same regardless of the composition of the electrode in Fig. 3. Referring to Fig.
• a .10 inch diameter button 40 is stamped from a sheet of rhodium foil having a thickness of 0.010 inches.
• a .005 inch diameter platinum wire 38 is spot welded to one surface of the disc 40 using molybdenum-copper and copper electrodes. The free end of the wire 38 is drawn into the bore of a glass tube 44 until the button 40 abuts the end of the tube. While maintaining the button 40 in abutment against the end of the tube 44, the tube is heated to a temperature above the softening point of the glass to cause the glass to flow around the button 40 and the wire 38 and the assembly is allowed to cool to room temperature. It is necessary to heat only that portion of the tube 44 adjacent the button 40 to achieve the desired fusion.
• the assembly is annealed at a temperature of 960°C for 12 to 14 hours to relieve any stress in the glass and then slowly cooled to room temperature.
• the button 40 is substantially encased in the glass. Accordingly, the end of the tube 44 is ground to expose the outer surface of the button 40 and to produce the desired radius of curvature and smoothness of the completed electrode 32 as illustrated in Fig. 3.
• capillary tubes in the manufacture of the fused glass electrode assemblies of Shis invention, that is, glass tubes having an O.D. of up to about 9 mm. and an I.D. of up to about 2 mm. Good results are achieved using capillary tubes having an O.D. of 6 mm. and an I.D. of about 1 mm.

Landscapes

• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Power Engineering (AREA)
• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Biochemistry (AREA)
• Physics & Mathematics (AREA)
• Analytical Chemistry (AREA)
• Electrochemistry (AREA)
• General Health & Medical Sciences (AREA)
• General Physics & Mathematics (AREA)
• Immunology (AREA)
• Pathology (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Molecular Biology (AREA)
• Microelectronics & Electronic Packaging (AREA)
• Measuring Oxygen Concentration In Cells (AREA)

Applications Claiming Priority (2)

Publications (2)

Family

ID=25139081

Family Applications (1)

Country Status (2)

Families Citing this family (1)

Citations (2)

Family Cites Families (20)

• 1986
  • 1986-10-03 EP EP19860906550 patent/EP0243445A4/fr not_active Withdrawn
  • 1986-10-03 WO PCT/US1986/002085 patent/WO1987002461A1/fr not_active Ceased

Patent Citations (2)

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events